Plate package, method of manufacturing a plate package, use of a plate package and plate heat exchanger comprising a plate package

ABSTRACT

The invention refers to a plate package for a plate heat exchanger, a method for manufacturing a plate package, a use of a plate package, and a plate heat exchanger. The plate package includes a plurality of heat exchanger plates ( 1 ), which are stacked on each other and which each includes a number of portholes. The plates ( 1 ) are compression molded and permanently connected to each other in a number of joints in such a manner that the plates between each other form a first passage for a first fluid and a second passage for a second fluid. The plate package is designed to permit at least one of the fluids to flow through the respective passages at a predetermined maximum working pressure. The plate package has an increased strength achieved by subjecting at least one of the passages to at least a local inner plastic deformation of the plate package.

BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention refers to a plate package for a plate heatexchanger, which package includes a plurality of heat exchanger plates,which are stacked on each other and which each includes a number ofportholes, wherein the plates are compression moulded and permanentlyconnected to each other in a number of joints in such a manner that theplates between each other form a first passage for a first fluid and asecond passage for a second fluid, wherein the plate package is designedto permit at least one of said fluids to flow through the respectivepassages at a predetermined maximum working pressure. The invention alsorefers to a method for manufacturing a plate package for a plate heatexchanger, which package includes a plurality of heat exchanger plates,which are stacked on each other and which each includes a number ofportholes, wherein the plates are compression moulded in such a mannerthat the plates in the plate package between each other form a firstpassage for a first fluid and a second passage for a second fluid andwherein the plate package is designed to permit at least one of saidfluids to flow through the respective passages at a predeterminedworking pressure. Furthermore, the invention refers to a use of a platepackage, and a plate heat exchanger.

Such plate packages are used in plate& heat exchangers for a pluralityof various applications. The plates are normally manufactured instainless steel and permanently connected to each other by brazing. Asbraze material copper is normally used. Such plate packages and plateheat exchangers have very high explosion pressures, i.e. they withstandvery high inner pressures in one or several of the passages withoutbraking of the plate package. The high explosion pressures are achievedthanks to the high ductility of the used materials and the capability ofthe materials to obtain a high yield limit through cold working. Theexplosion pressure may also be increased by increasing the sheetthickness of the heat exchanger plates, the pressure plate and the frameplate.

There is of course a general interest of increasing the strength of suchplate packages. In addition, in such plate packages, a certain spread inthe pressure fatigue quality arises since the permanent connectionbetween adjacent plates in certain joints could be defect or possible bepartly missing. During use of a plate package in a heat exchanger, theplate package is frequently subjected to pulsating pressure, wherein thehighest pressure pulses define, and are not permitted to exceed, thehighest permitted working pressure. These high pressure pulses lead tohigh stresses in such defect joints and in joints around defect jointsor around areas where the joints are partly missing for any reason. Highstresses are of course also present in all highly loaded areas even ifthe joints are free from defects.

During the manufacture of such plate packages, a pressure test of theplate package takes place today before delivery thereof. Such a pressuretest typically takes place at a test pressure corresponding to 1,3-1,8×the maximum working pressure depending on pressure vessel code,operation conditions, i. e. the strength of the material at the designedtemperature in relation to the strength at the test pressuretemperature. If the plate package withstands this pressure, the qualityis regarded to be sufficient. The test pressure level is such that itdoes not give rise to any visible or measurable plastic deformation ofthe materials in the-plate package.

U.S. Pat. No. 3,458,917 discloses a way of manufacturing another type ofplate heat exchanger. Two substantially plane plates are laid adjacentto each other and joined to each other at point- or line-shaped weldjoints. Thereafter a deformation pressure is supplied by the supply of apressurised medium to the interspace between adjacent plates. Thedeformation pressure is such that the plates will be deformed and obtaina wavy shape. In such a way, the desired passage between the plates iscreated.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a plate package for aplate heat exchanger with an increased fatigue strength.

This object is achieved by the initially defined plate package, which ischaracterised in that it has an increased strength achieved bysubjecting at least one of said passages to at least a local innerplastic deformation.

By such a prepressing the stresses in both the plates and the jointswill increase, and it is possible to exceed the yield limit of thematerial in certain points or areas. In these points or areas a plasticdeformation thus arises, which leads to a decrease of the stress inthese points or areas after the pressure again has been lowered to anormal level. During use of the plate package, these points or areaswill be subjected to a reduced load, and adjacent points or areas willabsorb a larger part of the load that arises. After the prepressingaccording to the present invention, it is thus possible to obtainfavourable rest stresses in the form of pressure stresses in the mostloaded area of the plate package. The plate package may for instanceinclude the materials copper and stainless steel, which have a highductility, and if these materials are used in the plate package thedesired condition with an equalised inner stress in the plate package isthus obtained prior to failure of the plate package in the most loadedpoint or points.

By the prepressing according to the invention, a plastic deformation isthus aimed at, but this deformation does not need to be as large that itis recognisable with the naked eye. After such a prepressing of theplate package, it is however possible to study the plastic deformationby means of any appropriate analysing equipment in order to establishthat a plastic deformation has taken place. After the prepressing, theplate package may also be analysed in order to discover directly ifcertain joints are defect or missing by studying the plastic deformationaround the joints. The plastic deformation may take place in the plateand/or in the material by which the plates are connected to each other.

By the proposed prepressing, it is possible to prolong the life of theplate package at pressure cycling to a low cost. For certain products,which already today have a sufficient life, it is instead, thanks to theinvention, possible to dispense with a frequently used pressure plateagainst which the plate package is arranged.

Furthermore, by the prepressing according to the invention a smallerspread in the life due to pressure fatigue is achieved. The equalisationof stresses inherent in the plate package leads to the advantage thatpulsating pressures and stress peaks arising during use of the platepackage are equalised and distributed over a larger part of the platepackage.

According to an embodiment of the invention, said plastic deformation isachieved by expanding the plate package from inside, i. e. the platesare pulled in a direction from each other. Normally, said localdeformation will appear in the proximity of said portholes. The areaaround the portholes is a critical part of a plate package of a plateheat exchanger. It is therefore advantageous if a plastic deformationmay be obtained in or in the proximity of the porthole. At least one ofsaid joints of adjacent plates are located in the proximity of saidporthole, and thus said local deformation in an advantageous manner isprovided at and/or in this joint. The deformation may refer to amaterial by which the joint is made and/or the material in the plates.

According to a further embodiment of the invention, said deformation isachieved by pressurising a supplied medium to a treatment pressure whichsignificantly exceeds the working pressure. By such a pressurising, theplastic deformation may be achieved in a simple and controllable manner.

According to a further embodiment of the invention, the treatmentpressure exceeds the working pressure by a factor that is at least 2.

Preferably the treatment pressure exceeds the working pressure by afactor that is at least 3, and more preferably by a factor that is atleast 4.

According to a further embodiment of the invention, the plates areconnected to each other by brazing.

The object is also achieved by the initially defined method, which ischaracterised by the steps of: attaching the plates to each other in anumber of joints in such a manner that a permanent joining of the platepackage is achieved, and providing at least one local inner plasticdeformation of the plate package in such a manner that the plate packageobtains an increased strength.

The object is also obtained by a use of a plate package manufacturedaccording to this method, a use of a plate package as defined above, anda plate heat exchanger including a plate package as defined above.

BRIEF DESCRIPTION OF THE DRAWING

The present invention is not to be explained more closely by adescription of various embodiments and with reference to the drawingsattached.

FIG. 1 discloses schematically a side view of a plate heat exchangeraccording to the invention.

FIG. 2 discloses schematically a heat exchanger plate for a platepackage according to the invention.

FIG. 3 discloses schematically a sectional view of a plate packageconstructed of heat exchanger plates according to FIG. 2.

FIG. 4 discloses schematically a sectional view of an area around a portchannel of the plate package in FIG. 3.

FIG. 5 discloses schematically a side view of a plate heat exchangermanufactured according to an alternative method.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

FIG. 1 discloses a plate heat exchanger including a plate package, whichincludes a plurality of heat exchanger plates 1. Such a plate 1 isdisclosed more closely in FIG. 2. The plate 1 is manufactured of aductile material, preferably stainless steel. In the embodimentdisclosed, each plate 1 includes four portholes 2′, 2″. Each plate 1includes a main extension plane p and a corrugation 3 of ridges andvalleys. The corrugation 3 has been achieved by compression moulding theplates 1. In the embodiment disclosed, the ridges and valleys extendaccording to a herring-bone pattern, i. e. they are inclined by adetermined angle to a longitudinal centre axis x of the plate 1.

Substantially all plates 1 in the plate package are identical and theportholes 2′, 2″ form four port channels extending through the platepackage. The plates 1 are however stacked on each other in such a waythat the corrugation 3 of every second plate points in a first directionand every other plate in a second opposite direction. In FIG. 2, itappears by continuos lines how the corrugation 3 extends in the plate 1.By dotted lines, the corrugation 3 of an adjacent rotated plate isindicated. The continuous and dotted lines represent a ridge and/or avalley depending on from which direction they are seen. When the plates1 are stacked on each other in this alternating order, the valleys ofthe plate 1 in FIG. 2 will be supported by the ridges of the mostclosely underlying plate 1. In such a way, a plurality of support pointsare obtained between the plates 1. Furthermore, the area most closely tothe portholes 2′ is located at a level which corresponds to a valley ofthe corrugation 3, compare FIG. 4, whereas the area 5, which is locatedmost closely to the porthole 2″, is located at a level corresponding toa ridge of the corrugation 3. When every second plate 1 is rotated 180°and the plates 1 are stacked on each other, the area 4 will thus abutthe area 5 of the closest underlying plate, which appears from FIG. 4.Each plate 1 also has a sloping edge area 6 and a flange 7 extendingoutwardly from a lower part of the edge area 6 in a plane which issubstantially in parallel with the main extension plane p of the plate1.

During the manufacturing of the plate package, the plates 1 are thusstacked on each other in the alternating order mentioned above. A folioor a paste of a braze material including a suitable metal or metal alloyis applied between each plate. In the embodiment, disclosed copper isused. The plates 1 are kept together and the plate package is heated tothe melting temperature of the braze material during a suitable periodof time. The plates 1 will then be connected permanently to each other,and joints 10, 10′ between the ridges and valleys of the corrugations 3,joints 11 around the portholes 2′, 2″ between the areas 4 and 5, andjoints 12 between the sloping edge areas 6, are formed.

As appears from FIGS. 3 and 4, interspaces are formed between adjacentplates 1 in the joined plate package. The interspaces form a firstpassage 15 between two of the portholes 2′ and a second passage 16between the two other portholes 2″. The plates 1 are arranged in such away that every second interspace is associated to the first passage 15and every other interspace to the second passage 16. The first passage15 is intended for a first medium and a second passage 16 for a secondmedium. The plate package and the plate heat exchanger are designed topermit a highest working pressure for the first medium and/or the secondmedium.

When the plate package has been brazed, as defined above, at least oneof the passages 15, 16 is subjected to a prepressing at a treatmentpressure which significantly exceeds the working pressure. FIG. 1discloses schematically an equipment for such a prepressing of one ofthe passages 15, 16. It is to be noted that both the passages 15, 16 maybe prepressed by such an equipment. In the latter case, the prepressingof both the passages may be made simultaneously or successively.

The equipment includes a pump 30 or the like which is connected to oneof the passages 15 and 16 via a pipe connection 31. The passage 15, 16in question is closed at the other end by means of a cover 32 or thelike, which is provided on a pipe connection 33 to the passage 15, 16 inquestion. By means of the pump 30, a medium from a source 35 is suppliedinto the first passage 15 or the second passage 16. Said medium may beany suitable gas or liquid. Suitably, a pressure sensor 36 is providedin the pipe connection 31 for enabling reading and/or recording of theapplied treatment pressure by means of a display member and/or arecording member.

The treatment pressure is chosen in such a way that it produces at leastone local inner plastic deformation in the plate package. By innerplastic deformation it is referred to a deformation in the material inthe plates 1 and/or any of the braze joints 10, 10′, 11, 12. Such aninner plastic deformation does not need to be visible with the nakedeye, but may be established by any suitable material analysingequipment.

The braze joints 10′, 11 around the portholes 2′, 2″ are particularlycritical since these areas lack the plurality of support points presentin the corrugated area of the plate. It is therefore desirable toproduce such a plastic deformation of the joints 10′ and the platematerial in the proximity of the portholes 2′, 2″.

The treatment pressure should significantly exceed the working pressurementioned above, for instance by a factor that is at least 2, preferablyat least 3 and specifically at least 4. By such a treatment pressure, apressure is ensured, which results in exceeding of the yield limit ofthe material and thus in an initiating of a plastic deformation. Thetreatment pressure thus depends on the maximum working pressure. Forinstance, the treatment pressure may in absolute numbers amount to atleast 20 bars, at least 30 bars, at least 40 bars or at least 50 bars.In certain applications where the maximum working pressure is muchhigher, the treatment pressure may be even higher than the levelsdefined.

Normally a plate package or a plate heat exchanger is pressure testedprior to use. The test pressure is about 1,3-1,8× the maximum workingpressure. Since the treatment pressure significantly exceeds such a testpressure, it is according to the invention possible to dispense withsuch a pressure test according to the prior art. A pressure test mayhowever also be performed by the supply of a medium pressurised to adetermined test pressure, which exceeds the working pressure but whichsuitably is lower than the treatment pressure.

FIG. 5 discloses an alternative way of producing the desired plasticdeformation. A piston 40 is introduced into a port channel via a pipeconnection 33. A holding-up member 41 is attached around the pipeconnection 33, and thereafter a pressure force is applied to the piston40 by means of for instance a hydraulic or pneumatic cylinder 42. Theforce will act against the outermost pressure plate 1′, and the plates 1will be pulled apart in a similar manner as if a supplied medium wouldhave been pressurised.

The invention is not limited to the embodiments disclosed but may bevaried and modified within the scope of the following claims. Theinvention is also applicable to plate packages and plate heat exchangerswhere the plates are permanently connected to each other in anothermanner than by braze joints, for instance by welding. A plate packagemay also be designed in such a way that the interspaces between theplates 1 form three or several separate passages. In particular, whensuch a plate package includes a so-called partition wall between twopassages large loads arise in the port area close to the partition wall.

1. A method for manufacturing a plate package for a plate heatexchanger, said package comprising a plurality of heat exchanger plates,which are stacked on each other and which each includes a number ofportholes, wherein the plates are compression molded in such a mannerthat the plates in the plate package between each other form a firstpassage for a first fluid and a second passage for a second fluid andwherein the plate package is designed to permit at least one of saidfluids to flow through the respective passages at a predeterminedworking pressure, comprising: attaching the plates to each other in anumber of joints in such a manner that a permanent joining of the platepackage is achieved, and providing at least one local inner plasticdeformation of the permanently attached plate package in such a mannerthat the plate package obtains an increased strength.
 2. A methodaccording to claim 1, wherein said plastic deformation is achieved byexpanding the plate package from inside.
 3. A method according to claim1, wherein said local deformation appears in the proximity of saidportholes.
 4. A method according to claim 3, wherein adjacent plates areattached to each other along at least one common joint which is locatedin the proximity of said portholes, and wherein said local deformationis provided at and/or in said joint.
 5. A method according to claim 1,wherein said deformation is provided by the application of a force whichpulls apart the plates.
 6. A method according to claim 1, wherein saiddeformation is provided by the supply of a medium, which is pressurizedto a treatment pressure that significantly exceeds the working pressure,to a least one of said passages.
 7. A method according to claim 6,wherein said medium is pressurized to a treatment pressure that exceedsthe working pressure by a factor that is at least
 2. 8. A methodaccording to claim 6, wherein said medium is pressurized to a treatmentpressure that exceeds the working pressure by a factor that is at least3.
 9. A method according to claim 6, wherein said medium is pressurizedto a treatment pressure that exceeds the working pressure by a factorthat is at least
 4. 10. A method according to claim 5, wherein saidforce is supplied by means of a piston which is introduced into a portchannel in the plate package.
 11. A method according to claim 1, whereinthe plates are attached to each other by brazing.
 12. A method accordingto claim 1, further comprising: supplying a medium which is pressurizedto a determined test pressure that exceeds the working pressure but islower than a treatment pressure.